Nanosize powders have numerous applications such as catalysts, electrocatalysts, catalyst supports, electrodes, active powders for the fabrication of dense bodies, semiconductors for energy storage, photovoltaics, ultra-fine magnetic materials for information storage, environmental clean-up as destructive adsorbents, water purification, information storage, and optical computers, to name a few. Some of the numerous examples include the following: nanosize (3 to 4 nm) platinum for oxygen reduction in acid electrolytes, many metallic powders made by precipitation in aqueous and non-aqueous media for alloy fabrication and for catalysis, nanosize iron oxide catalyst for coal liquefaction, nanosize iron oxide particles for magnetic applications, tetragonal zirconia powder by a hydrothermal treatment at high pressures for structural applications, carbides and nitrides using non-aqueous media, nanosize BaTiO3 by a gas-condensation process, etc. Many oxides have potential applications as nanosize powders. Some useful oxides include CeO(2-x) for catalytic reduction of SO2, γ-alumina as a catalyst support and for enhancing ionic conductivity of lithium iodide, V2O5 as a catalyst for NOx reduction, and etc. Several processes currently used for the synthesis of nanosize powders include gas-phase condensation, mechanical milling, thermal crystallization, chemical precipitation, sol-gel processing, aerosol spray pyrolysis, and the like.
In gas-phase condensation, evaporation of precursors and their interaction with an inert gas leads to loss of kinetic energy such that homogeneous nucleation of nanosize powders occurs in a supersaturated vapor. Nanocrystalline powders of TiO2, Li2O-doped MgO, CeO2, Y-doped ZrO2, etc. have been produced by gas-phase condensation. Aerosol spray pyrolysis has been used to synthesize BaFe12O19, Fe2O3 among some other materials. High-energy mechanical milling is used extensively to produce nanostructured materials, especially when large quantities of materials are required. Very fine particles of nickel-aluminum alloy, Fe—Co—Ni—Si alloys, Ni—Mo alloys, for example, have been produced by mechanical milling. Contamination by the milling process, however, is a shortcoming of this process. Also, although very fine (nm size) particles can be made, agglomeration is a problem leading to cluster sizes in the micron range.
Chemical co-precipitation has received considerable attention for the synthesis of nanosize powders. Metallic as well as ceramic powders can be made by a careful control of the precipitation chemistry. Alkali metal borohydride, MBH4 where M is an alkali metal, for example, has been used as a reducing agent in aqueous media for the synthesis of metallic powders. Similarly, hydroorganoborates of the general formula MHv(BR3) or MHv[BRn(OR′)3-n]v where M is an alkali or alkaline earth metal, v=1 or 2, and R, R′ are alkyl or aryl groups have been used as reducing and precipitating agents. It is important to control pH and ionic strength in aqueous media to prevent Ostwald ripening. In the synthesis of nanosize iron oxide, for example, it has been shown that the higher the pH and the higher the ionic strength, the smaller the size of nanosize Fe3O4 particles.
In most methods for the synthesis of nanosize powders, two issues are particularly important. First, the formation of fine uniform size particles, and second, the prevention of agglomeration are important considerations in nanosize particle synthesis. In principle, nanoparticles of a uniform size can be formed by carefully controlling nucleation and growth. Often, a variety of encapsulating methods is necessary to control the size of nanoparticles.
Agglomeration is often the result of Van der Waals forces. The adverse effect of agglomeration on the sintering behavior of ceramic powders is well documented. Even in catalysis, the need for dispersed powders is well known. Often, supercritical drying can be used to obtain non-agglomerated powders. In liquid media, agglomeration can be suppressed through steric hindrance or through the manipulation of electrostatic interactions. The latter in polar liquids can be achieved by changing the pH and the ionic strength of the solution. Many agglomeration suppression techniques involve the use of surfactants. Often, the powders which are non-agglomerated and well dispersed in a liquid, tend to agglomerate during the drying stage. Fortunately, methods such as slip-casting, gel-casting and pressure slip casting can be used to achieve powder compaction in a wet state. Such has been demonstrated using submicron ceramic powders.
With the exception of milling, all the above methods are based on molecular synthesis of nanoparticles wherein the particles are built-up by atom-by-atom, or molecule-by-molecule, addition. Even in processes based on the decomposition of metal carbonyls, the growth of particles occurs via a layer-by-layer addition of atoms. As a result, a control of nucleation and growth is necessary to ensure the formation of nanosize particles. This often requires a very precise and difficult control of the reaction system, which renders the manufacture of the nanosize powder in large quantities impractical or impossible. In addition, the molecular synthesis processes are costly because of relatively large capital expenditures required for the equipment to control the formation of only a small quantity of nanosize product. As such, devices and methods for producing nanosize materials continue to be sought through on-going research and development efforts.
It is, therefore, an object of the present invention to provide methods for the formation of nanosize powders that are easy to implement on an industrial scale and is relatively inexpensive when compared to molecular synthesis methods. Another object of the invention is to provide methods in which nanosize powders are formed by a process other than precipitation or deposition from solutions, thus eliminating the possibility of unwanted deposition and growth of the nanosize powders.